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Ion-projection Lithography (IPL)

Ions can also be projected through stencil masks.¹ Because ions scatter very little in solids, they can potentially result in very high resolution. However, because they are much more massive than electrons (the lightest ion, H⁺, is approximately 2000 times more massive than an electron), ions cannot be deflected at the same speed as electrons. This is a consequence of basic physics:

A much larger force F is required to accelerate an ion than to accelerate an electron. Consequently, the deflection schemes that have been proposed for SCALPEL cannot be easily adopted for ion-beam exposures. For this reason, large-field (12.5 mm × 12.5 mm) ion-projection lithography systems are being pursued.

A schematic of an IPL system is shown in Fig. 12.20. Ions with a small spread in energy are produced by a recently developed ion source.^2,3 Electrostatic lenses are then used to produce a uniform beam of ions that covers the area of the mask. Beam energies are typically = 250 keV. Another set of electrostatic lenses then reduces the size of the overall pattern 4× and focuses the ions onto the wafer surface. An example of electrostatic lenses is shown in Fig. 12.21.

Ion-projection lithography requires stencil masks⁵; there is no potential for a membrane mask option, unlike with electron projection lithography. One of the problems with stencil masks is the donut problem. Consider the mask shape shown in Fig. 12.22. This cannot be made with a single stencil mask, as the center portion will be unsupported and will fall out. Creation of the geometry shown in Fig. 12.22 requires the use of at least two masks. While not fundamentally limiting, the need for double exposures will reduce throughput on some layers.

Because ion masks must block ions in order to generate masked patterns, there is considerable energy deposited into the masks. With ion beams, there is also the potential for sputtering of the masks. Work has shown that deposition of a carbon film provides resistance against ion damage and high emissivity to enable radiative cooling of the ion-projection masks.⁶

References

1. G. Stengel, H. Loschner, W. Maurer, and P. Wolf, “Current status of ionprojection lithography,” Proc. SPIE 537, pp. 138–145 (1985).
2. Y. Lee, R.A. Gough. K.N. Leung, J. Vujic, M.D. Williams, and N. Zahir, “Plasma source for ion and electron beam lithography,” J. Vac. Sci. Technol. B 16(6), pp. 3367–3369 (1998).
3. K. Leung, “Plasma sources for electron and ion beams,” J. Vac. Sci. Technol. B 17(6), pp. 2776–2778 (1999).
4. E. Spiller, S.L. Baker, P.B. Mirkarimi, V. Sperry, E. M. Gullikson, and D. G. Stearns, “High-performance Mo-Si multilayer coatings for extreme-ultraviolet lithography by ion-beam deposition,” Appl. Opt. 42(19), pp. 4049–4058 (2003).
5. A. Ehrmann, A. Elsner, R. Liebe, T. Struck, J. Butschke, F. Letzkus, M. Irmscher, R. Springer, E. Haygeneder, H. Löschner, “Stencil mask key parameter measurement and control,” Proc. SPIE 3997, pp. 373–384 (2000).
6. P. Hudek, P. Hrkut, M. Drzik, I. Kostic, M. Belov, J. Torres, J. Wasson, J. C. Wolfe, A. Degen, I.W. Rangelow, J. Voigt, J. Butschke, F. Letzkus, R. Springer, A. Ehrmann, R. Kaesmaier, K. Kragler, J. Mathuni, E. Haugeneder, and H. Löschner, “Directly sputtered stress-compensated carbon protective layer silicon stencil masks,” J. Vac. Sci. Technol. B 17(6), pp. 3127–3131 (1999).